1. Field of the Invention
This invention relates to a liquid thermosetting resin composition, particularly a liquid thermosetting resin composition which is useful as a composition for permanently filling such holes as via holes and through-holes in printed wiring boards or printed circuit boards such as multi-layer boards and double-sided boards. The liquid thermosetting resin composition of the present invention may also be used as various resists, interlaminar insulating materials, sealing compounds for IC packages, and the like.
The present invention further relates to a printed wiring board whose holes such as via holes and through-holes have been permanently filled by the use of the composition mentioned above and a process for the production thereof.
Incidentally, the term “hole part” as used in this specification refers collectively to a via hole, a through-hole, and the like formed in the course of production of the printed wiring board.
2. Description of the Prior Art
In recent years, the printed wiring boards have been advancing toward decreasing the width of pattern lines and decreasing a mounting area. Further, with a view to coping with the trend of devices provided with a printed wiring board toward decreasing size and increasing function, the desirability of producing printed wiring boards in further decreased weight and size has been finding growing recognition. As a result, the printed wiring boards are advancing in the direction of favoring the build-up process which comprises superposing resin insulating layers one each on the upper and the lower surface of a core substrate, forming necessary conductive circuits thereon, further repeating the superposition of resin insulating layers and the formation of conductive circuits, and so on and the mounting parts are advancing in the direction of favoring such an area array type as, for example, BGA (the ball grid array) and LGA (the land grid array). In such circumstances, the desirability of developing a composition to be used in permanently filling the hole parts such as through-holes and via holes and excelling in filling properties, polishability, and physical properties of a hardened product has been finding universal recognition.
As the composition for permanently filling such holes in printed wiring boards, in general the epoxy resin compositions of the thermosetting type have been widely used because their hardened products excel in mechanical, electrical, and chemical properties, and exhibit good adhesiveness as well.
Further, the process for permanently filling holes in a printed wiring board by the use of such an epoxy resin composition comprises in general a step of filling the hole parts of the printed wiring board with the epoxy resin composition, a step of heating the composition filled in the hole parts thereby effecting precure of the composition to a polishable state, polishing and removing the parts of the precured composition which protrudes from the surface defining the hole parts, and further heating the precured composition till final curing.
In this process for permanently filling holes in a printed wiring board, when the hole parts such as through-holes and via holes of the printed wiring board are filled with the epoxy resin composition, the filled composition inevitably gives rise to voids due to such phenomena as air entrapment. It was hardly possible to completely remove or vanish such voids even when the precure and the final cure are performed. Such phenomena were remarkable in proportion as the depth of the hole part increases (in the case of the through-hole, as the thickness of the core substrate increases) and as the viscosity of the epoxy resin composition becomes higher. Further, in the process for permanently filling holes in a printed wiring board mentioned above, there was another problem of the occurrence of cracks during the step of precure.
Moreover, there was another problem that the composition shrinks largely during the course of thermal curing and the finally cured composition gives birth to a gap between itself and the wall of the through-hole.
When such defects as voids and cracks occur in the cured products of the resin composition filled in the hole parts in a printed wiring board, these parts exhibit hygroscopicity, and the printed wiring board suffers from the decrease of PCT resistance (pressure cooker resistance) under the conditions of high temperature and high humidity and brings about the occurrence of cracks and deterioration of insulation reliability during the course of heat cycle of the printed wiring board. Moreover, these defects also becomes the cause of ply separation of the insulating resin layer or cover plating formed on the cured products filled in the hole parts.